Digital volt meter



J. E. DEAVENPORT ETAL 3,375,351

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United States Patent @hice 3,375,351 Patented Mar. 26, 1968 3,375,351 DIGITAL VOLT METER Joe E. Deavenport and Don W. Sexton, San Diego, Calif., assignors, by mesne assignments, to Weston Instruments, Inc., Newark, NJ., a corporation of Delaware Original application Apr. 3, 1963, Ser. No. 270,336, now Patent No. 3,327,228, dated June 20, 1967. Divided and this application `lan. 3, 1967, Ser. No. 619,107

3 Claims. (Cl. 23S- 92) This is a division of application Ser. No. 270,336, filed Apr. 3, 1963, now Patent Number 3,327,228.

This invention relates generally to electrical'systems and more particularly to such systems which convert electrical quantities, such as voltages, to cyclic or discrete electrical signals or other type of physical manifestation.

Efforts to accurately measure and indicate voltages have resulted in the ldevelopment of various types of converter systems. Such systems when including a facility for producing a numerical indication Vof the input quantity or voltage are usually referred to as digital volt meters. These instruments in a broad sense usually include a converter capable of converting the input quantity, that'is, voltage, to a numerical indication. For this purpose various types of readout devices may be employed, one'type being the conventional numerical wheel counter, and another, and preferred type of device, employs gas discharge tubes stacked adjacent one another in an envelope having a transparent end and covering a range of decimal numbers from `zero through 9. The use of pluralities of such number wheels or gas discharge tubes energized by suitable switching circuits controlled Iby the output of the converter circuits provides a numerical indication of the input voltage. The control of suchnumerical indicating devices by prior art converters has proved satisfactory in many applications, but 4where high-speed operation and high accuracy must be had prior art arrangements are unsatisfactory.

One prior `art arrangement utilizes stepping switches kwhich are connected in a bridge circuit. The bridge circuit is electrically unbalanced inan amount proportional to the voltage to be measured and the stepping switches which are energized by the bridge unbalance voltage are used to electrically balance the `bridge at which time the stepping switches stop. The electrical configuration of the stepping switches at this point is presumably indicative of the magnitude of the input voltage. The stepping switches may be utilized to selectively energize or control numerical read-out devices of the type referred to above.

In still another prior art arrangement, the speed of operation has been increased somewhat and noise reduced in the use of reed type relays. These relays have small, flexibly mounted contacts which are magnetically actuated. The use of pluralities of such Vreed relays in suitable circuit arrangements permits switching of the type provided by stepping switches so that the outputs of the reed relays may be used to control suitable numerical indicators.

Although this latter type of device is faster than the stepping switch type, it is still too slow for many applications and the accuracy of the conversion is not suitable for many applications.

Another type of converter circuit which has been employed utilizes a lsumming integrator which is c-ontrolled by an input voltage to control a pulse generator. The output of the pulse generator is then fed back in a negative sense to the input of the summing integrator. T-he use of a closed loop system insuch an arrangement offers some advantages with respect to linearity. However, the application of the input voltage -directly to the integrator requires that the integrator cycle at a rate proportional to the magnitude of the input voltage. Thisis accomplished by using a pulse forming network responsive to a predetermined magnitude of integrator output and forming output pulses. These output pulses are fed back to be re-cycled each time its output voltage reaches a predetermined summing junction at the'integrator input and drive the input circuit in a direction todrive the integrator output to zero. Thus, the cyclic rate of the integrator determines the pulse rate. Such an arrangement may be satisfactory at relatively low frequencies, but at higher frequencies linearity is not satisfactory.

One object of this invention is to provide an improved converter system.

Another object of this invention is to provide .an improved voltage to frequency converter.

lA specific object of this linvention is to provide an improved digital volt meter.

The aforesaid and other objects and advantages `are achieved in an arrangement according to the' present invention wherein a chopper stabilized potentiometric type of amplifier system is utilized to control a voltage controlled oscillator. The input to the potentiometric type of amplifier is preferably in the form of a voltage which is to be measured. The voltage controlled oscillator may be any suitable typeof oscillator which has an output voltage which is substantially linearly -related to its input voltage. T-he system is arranged so that at zero input volts the voltage controlled oscillator will have a particular output frequency which, in one practical embodiment of this invention, decreases with the application of a positive input voltage to the potentiometric amplifier system, and increases when a negative input voltage is applied to the potentiometric amplifier system. The system is further arranged to provide about 99 percent accuracy in the direct conversion of 4the input voltage to an output frequency at the output of the voltage controlled oscillator.

The output voltage of the po-tentiometric amplifier system is compared with a reference voltage, in this case a negative reference voltage, and the difference is coupled input-wise to an integrating amplifier. The output of the integrating amplifier, after suitable filtering and additional amplification, if needed, is coupled to the voltage controlled oscillator and provides the remaining 1 percent of regulation required to achieve linearity between the input voltage and the output frequency. f

Unlike the prior art devices the integrating amplifier of this invention is referred to ground and a negative precision reference voltage and is operated "at a vfrequency which is well below the frequency of the voltageV controlled oscillator. To this end,l the input and output circuits of the integrating amplifier are coupled to respective grounding switches forming part of a reset circuit which is controlled by the output of a frequency divider circuit in turn controlled by the output'of the voltage controlled oscillator. By this expedient the output frequency ofthe voltage controlled oscillator is divided to any selected lower frequency and the lower frequency utilized to periodically control switching of the resetting circuit to ground the input and output circuits of the integrating amplifier in the control loop.

In the application of the specic converter in a converter system to achieve a numerical indication of the input voltage the output of the voltage controlled oscillator is coupled to a gating circuit. This gating circuit is periodically switched and enabled by a suitable timing system including a crystal oscillator. The output of the crystal oscillator controls a timing counter which funcywhich the output of the voltage controlled oscillator is gated. The output of the gate is coupled to a suitable frequency counter. During the remaining part of each timing counter cycle the count in the frequency counter is transferred to a memory and read-out system which produces a numerical indication' of the magnitude of the input voltage.

y yInasmuch as the voltage controlled oscillator operates at a given frequency for zero input voltage, provision is made in the digitalcircuits to exhibit zeros on the numerical indicator for this particular condition and to further indicate the application of a positive voltage which reduces the frequency of the voltage controlled oscillator and a'negative voltage which increases the frequency of the voltage controlled oscillator. Additionally, provision is made for transferring the 9s complement of the number in the digital portio-n of the system for numerical readout purposes at such time as a positive voltage is applied to the input of the potentiometric amplifier system.

Inasmuch as a system of this type has nite capacity sfor indicating magnitudes of input voltage, provision may lbe made when voltages greater than those capable of indication Within the counting abilities of the system are applied to the input of the potentiometric amplifier system, and, provision made under the control of the digital portion of this system through suitable attenuators at the input to the potentiometric amplifier system to select attenuation values bringing the input voltage within the acceptable range.

The aforesaid and other objects and advantages will be better `understood by reference to the following specification when considered in conjunction with the accompanying drawings in which:

FIGURE l is a block diagram of a converter system embodying the principles of this invention;

FIG. '2 is a diagrammatic illustration of a chopper circuit employed in stabilizing the potentiometric amplifier system of the voltage to frequency convertor herein;

FIG. 3 diagrammatically illustrates the potentiometric amplifier system;

FIG. 4 diagrammatically illustrates the voltage controlled oscillator circuit of the voltage to frequency convertor;

A FIG. 5 diagrammatically illustrates an integrating amplifier circuit employed in the voltage to frequency convertor;

FIG. 6 diagrammatically illustrates another amplifier employed in this invention in the voltage to frequency convertor;

FIG. 7 diagrammatically illustrates a reset flip fiop employed in this invention forming part of a frequency divider in the voltage to frequency convertor;

. FIG.` 8 is a timing diagram illustrating several output voltages of thefrequency divider circuit;

FIG. 9'is a modification of the voltage to frequency convertor circuit illustrated in FIG. 1;

FIG. 10 graphically depicts certain output voltage characteristics of the integrating amplifier circuit of FIG. 9;

FIG. 11 is -a block diagram of the digital portion of the convertor system of this invention;

, FIGS. l2 and 13 are timing diagrams depicting operatingcharacteristics of several elements of the digital portion of the system of this invention for negative and` positive input voltages, respectively;

FIG. 14 is a block diagram illustrating a portion of a digital c-ounter employed in the digital system of this in- Vention;

FIG. 15 is a timing signal diagram depicting the typical operation of the flip flops of the respective decades of the counter of FIG. 14;

' FIGS. 16 and `17 diagrammatically illustrate typical counter flip flops;

FIG. 18 diagrammatically illustrates a polarity and range indicator circuit controlled by the counter and memory and read-out circuits;

FIG. 19 diagrammatically illustrates one memory and read-out decade of this invention and typically represents the other decade; and

FIG. 20 diagrammatically illustrates a range control circuit employed in this invention.

CONVERTOR SYSTEM Voltage to frequency convertor (general) Referring to FIG. 1, the convertor -system illustr-ated therein includes a convertor circuit for converting a particular input voltage to a corresponding frequency. The input voltage is provided by an input circuit I, generally illustrated-in block form, which is coupled to a terminal TE1 at the input of a chopper stabilized amplifier A11 constituting part of a potentiometric amplifier system including additionally an amplifier A12. Amplitier A12 is controlled by the output of the amplifier A11 and is additionallycontrolled by means of a booster circuit BC in accordance with the differential of input and feedback voltages from terminal TE1 and the output of amplifier A12, respectively, and having an output circuit coupled through a capacitor `C2 to the input of the amplifier A12 to increase transient response. The output of the amplifier A12 is coupled input-wise to an input terminal TES of a voltage controlled oscillator VCO having respective output circuits represented in terminals TE7 and TES.

As noted hereinabove, the input voltage circuit may be any suitable type of convertor capable of converting any physical condition to an output voltage, or may be any suitable voltage source.

A floated chopper drive circuit FC is coupled to input terminals TE2 and TES of the amplier A11 to drive the chopper which modulates the input voltage circuit at some predetermined frequency. Terminal TES of the amplifier A11 is also coupled to the output circuit of the amplifier A12 completing -a feedback voltage circuit around the potentiometric amplifier system. A feedback capacitor C1 may also be coupled -between the output and input circuits of the amplifier A12.

An integrating amplifier AZ having an integrating capacitor C3 has its input circuit coupled to a terminal TE9 forming part of a precision resistor network including a resistor R1 having one end coupled to the output circuit of the potentiometric amplifier system and further including a resistor R2 coupled to the negative reference voltage, here indicated -Vreb The output circuit of integrating amplifier A2 is coupled through a resistor R5 to a terminal TE9a in a voltage divider network between the feedback circuit of the potentiometric amplifier system and the negative reference voltage -Vmp This circuit includes the series connected precision resistors R3 and R4 and a trim resistor circuit including a resistor R311 having an adjustable tap R3b. Resistor R3 forms part 0f a linearity adjusting circuit providing controlled compensation of the output of integrating amplifier A2 in dependence upon the output from amplifier A12 to improve the operation.

The output of the integrating yamplifier A2 is filtered by means of a filter F the output of which in turn is coupled by an amplifier A31, which again is a chopper stabilized amplifier having its input circuits referenced to ground and being controlled by the output of the chopper drive circuit, being coupled to a terminal TE4 of the chopper coupled input-wise to the amplifier A31.

The output of amplifier A31 is coupled by means of an amplifier A32 to a control input terminal TEG of the voltage controlled oscillator which completes the control loop.

The integrator amplifier A2 has its input and Output circuits periodically grounded by means of a reset circuit, generally designated RC. This reset circuit comprises a pair of `switches S1 and S2, respectively, coupled to the input and output circuits. When switches S1 and S2 are closed the input and output circuits are connected to ground as indicated. Switches S1 and S2 in some embodiments may be mechanical types of switches. In accordance with this invention, however, transistors are contemplated as switching elements. As will |be described at a later point, pluralities of transistors are embodied in each of the switches S1 and S2 and, as will be described, they are inverted and used as switches to provide fast and positive grounding of the respective circuits.

Control of the switches S1 and S2 is achieved by means of a frequency divider, generally designated FD, cornprsing a reset counter RCO and a reset fiip fiop RFC. The reset counter may be any suitable type of counter but as employed herein embodies a plurality of bistable liip fiops conventionally cascaded by coupling each output circuit to the next higher order input circuit to achieve conventional binary operation. Such a counter may comprise 1l) cascaded flip iiops FR.1 through FRI()7 as indicated, in which the output circuits R87, RSS, 89 and R810 are coupled input-wise to the reset fiip op circuit, to control the lreset ip iiop circuit to produce a pulse in its output circuit R811 once during each counting cycle of counter RCO. The output terminal ISll is connected to the switches S1 and S2 to periodically operate these switches to ground the input and output circuits of the amplifier A2. In View of the connection of the frequency divider circuit FD with the voltage controlled oscillator VCO to `be controlled thereby, it will be seen that the switching rate or period of the switch S2 is directly controlled yby the frequency of the voltage controlled oscillator, the period being longer when the frequency of the voltage controlled oscillator is lower and being shorter as the voltage controlled oscillator output frequency increases. Thus the output voltage of the integrator amplifier is a function of |both its input and the time interval during which it operates. This will be understood from the following explanations:

When there is no input voltage coupled to the system, there is a substantially constant current input to the integrator amplifier A2 provided by the voltage reference ref and the resistor R2. This current is proportional to the center frequency of the output of the voltage controlled oscillator VCO. Since the reference voltage is a negative voltage, the output of integrator amplifier A2 will be a positive going voltage ramp at .a voltage rate determined by the reference Voltage and by the values of the resistor R2 and the capacitor C3. This output voltage of the integrator amplifier A2 has an average DC potential that is balanced to ground with resistors R5 and R4 and which is further amplified by the amplifier A31 after filtering. Since there is no input to the amplifier A11, the frequency output of the voltage controlled oscillator VCO is stabilized to a point where the average DC voltage out of the integrator amplifier A2 is effectively balanced out in resistors RS and R4, providing a virtual zero voltage input to the amplifier A31 (assuming amplifier A31 is a very high gain amplifier). If the output frequency of the voltage controlled oscillator should be too high, the switches S1 and S2 will reset the output of integrator amplifier A2 in a shorter period of time and there will be less positive DC average voltage from the output of integrator amplifier A2. This represents a negative input to the amplifier A31 (which is referenced to ground) and which will therein be amplified to give a positive input to the voltage controlled oscillator VCO to lower its frequency and correct the error. If an input voltage is applied to the amplifier All of a positive polarity, a positive going voltage is applied to the input circuit of the voltage controlled oscillator. This results in a lower frequency output providing a longer period of time between the switching cycles of the switches S1 and S2 and consequently a longer period of time between the times when the integrator amplifier A2 is reset. But now the input current to the integrator amplifier A2 will be decreased by the current through resistor R1 which is opposite to the current through resistor R2, and the ramp generated at the output of integrator amplifier A2 will rise at a slower rate, i.e., a lower voltage per second slope characteristic will exist and the average area or average DC output from integrator amplifier A2 will tend to remain constant, if the ratio of the system input Voltage to the output frequency of the voltage controlled oscillator is in the desired ratio.

Digital counter (general) The output terminal TES of the voltage controlled oscillator is coupled to a gating circuit, generally designated G. As will be explained hereafter, such a gating circuit may be a transistor gate which is enabled at such time as a signal POS is in the lower of its two voltage states, and disabled at such time as the 4signal POS is in the higher of its two voltage states. The signal FOS is generated by a flip fiop FP() forming part of a programmer, generally designated P and which includes additionally flip flops FP1, FP2 and FPS.

The programmer P is controlled by means of a timing counter, generally designated TC, having a first output driving circuit DRI, coupled input-wise to control the iip flops of the programmer P, as will be described in greater detail hereafter, and having additionally a control output circuit by means of which the signals T14s or TMS are coupled to the programmer. The control signals T14s and TMS control the counting and operating intervals of the digital system.

rThe timing counter TC establishes system timing and for this purpose it must operate in precise time intervals. To this end the timing counter is controlled or driven by means of a crystal oscillator, generally designated XO which operates at some fixed frequency, say of the order of l0() kilocycles, and has an electrical output directly coupled to the timing counter. As will be explained hereinafter, the timing counter comprises a plurality of flip flops FT|1 through FT14 and functions essentially as a frequency divider in producing the several electrical outputs which are indicated.

The output of the gating circuit G is coupled as the count input to the input circuit of a counter represented as a block, generally designated CO. As will be explained, this is a decimal counter and comprises four complete 4-ip flop decades producing respective groups of signals CO-l through CO-4 and O O-1 through 'OO-4. These signals in any suitable binary code indicate the number of pulses applied to the input circuit of the counter during the interval in which the gate G is enabled. Thus, the output of the counter is a binary number representative of the magnitude of the input voltage.

These number indicating signals are coupled inputwise to a memory and read-out circuit indicated as a block and generally designated MR, which stores the output of the counter circuit. The memory and read-out circuit also includes a suitable read-out device such as a numerical indicator of the type described hereinabove which is coupled to and controlled by the storage circuits. The details of these circuits will be explained at a later point. The counter CO includes two additional fiip ops FCSl and FC52 in a final or highest order decade. These fiip flops produce outputs S-l, C5-2 and S-Z which are voltage state signals. When flip-Hop FCSi is in its l representing electrical state, this indicates a positive input voltage has been applied to the input circuit I. When flip flop FCSZ is also in its 1 representing electrical state, this indicates that a negative input voltage has been applied to the input circuit I.

These flip flop signals are used along with a number of 

1. A DIGITAL SYSTEM COMPRISING: VOLTAGE TO FREQUENCY CONVERTER MEANS HAVING INPUT CIRCUIT MEANS AND HAVING OUTPUT CIRCUIT MEANS, AND HAVING A PREDETERMINED OUTPUT FREQUENCY FOR ZERO INPUT VOLTAGE, SAID CONVERTER MEANS INCREASING IN FREQUENCY FOR INPUT VOLTAGE OF ONE POLARITY AND DECREASING IN FREQUENCY FOR INPUT VOLTAGE OF THE OPPOSITE POLARITY; A COUNTER COUPLED TO SAID OUTPUT CIRCUIT MEANS OF SAID VOLTAGE TO FREQUENCY CONVERTER MEANS AND HAVING A MAXIMUM COUNT SETTING IN RESPONSE TO SAID PREDETERMINED OUTPUT FREQUENCY; A PAIR OF SWITCHES COUPLED TO AND CONTROLLED BY SAID COUNTER; STORAGE CIRCUIT MEANS; TRANSFER CIRCUIT MEANS COUPLING SAID STORAGE CIRCUIT MEANS TO SAID COUNTER AND HAVING CONTROL CIRCUITS; AND CONTROL MEANS COUPLED TO AND CONTROLLED BY SAID SWITCHES AND HAVING OUTPUT CIRCUIT MEANS COUPLED TO SAID CONTROL CIRCUITS. 